1. Field of the Invention
The present invention relates to a group III nitride compound semiconductor laser diode (LD hereinafter). In particular, the present invention relates to an LD with improved oscillation efficiency.
2. Description of the Related Art
It has been known that an aluminum gallium indium nitride (AlGaInN) compound semiconductor may be used as a material of LD which emits blue or ultra violet light in short wavelength region. The semiconductor device is useful because of its high luminous efficiency resulting from direct electron transition and because of its ability to emit blue light, which is one of the three primary colors, or green light.
An AlGaInN compound semiconductor with p-type conduction can be obtained by doping magnesium (Mg) and irradiating an electron beam thereto or carrying out heat treatment thereto. As a result, an LD with a double hetero-junction structure which includes a p-type cladding layer (AlGaN), an active layer made of indium gallium nitride (InGaN), and an n-type cladding layer made of gallium nitride (GaN) is proposed.
Further, an LD with a separate confinement hetero-strucure (SCH) is proposed which separates region for confining carriers from that for confining light and additionally includes layers for confining light only because a conventional LD has difficulties in confining light within a thin active layer only. Namely, the LD with a SCH has an active layer, a pair of guide layers, and a pair of cladding layers. The active layer is formed between the pair of guide layers. The pair of guide layers are formed between the pair of cladding layers. Carriers are confined within the active layer and light is confined within the active layer and the pair of guide layers.
In order to increase the output power of the LD with SCH, it is necessary to increase amount of carrier injected into the active layer. It, however, incurs another problem that carriers cannot be confined within the thin active layer and overflow into the guide layers. As a result, output power of the LD is not high enough.